Figure 1. Cross-row burners with water shields used for flame weed control.

Figure 2. Flame weed control in operation. Note that burners are staggered and aimed across the bed top.

Flame weed control can be an effective adjunct to cultivation for some vegetable crops and some weeds. Flame weed control fits especially well in organic operations.

What is Flame Weed Control?
Flame weed control uses heat to kill weeds selectively. The weeds are not burned; rather, the heat causes the cells in the weed plants to expand and rupture, thus causing the plants to wilt within 10-15 minutes and often die. The flames are provided by liquefied petroleum gas (LPG) – propane or butane.

Water Shielding
A critical component of a flame weeder is a flat fan spray nozzle mounted just above the burner. This nozzle will spray a fan of water just over the flame area. The water accomplishes two things: it helps shield the crop plants from the heat and it concentrates the heat on the weeds.

Equipment Setup
The normal configuration for a flame cultivator is to mount two flat burners on opposite sides of each bed, with both aimed at a low angle across the bed (Figure 1). The burners must be staggered and not aimed at each other or the flame will be forced up into the crop. It is essential that the location of the burner relative to the bed top be maintained, thus cone guide wheels are used for lateral guidance and slides for vertical guidance. Because of the high fuel flow rate required, the fuel tank should be of the liquid-feed type, not the vapor-feed type used for outdoor cooking. A solenoid valve that allows the operator to throttle back the gas flow without turning the burners off is useful. It will allow the burners to be cut back on turns without having to relight.

Getting Good Results
First, flame weed control is not the answer to every weed problem. Some succulent weeds such as purslane suffer minimal injury from flame. Also, some vegetable crops are more easily damaged by flame than others. Crops such as beans, peas and sweet corn seem to withstand flame with little damage. Finally, flame weed control absolutely will not work unless there is a height difference between the crop and the weeds. The crop must be taller than the weeds! You need a situation where the flame passes under the crop leaves but hits the weed leaves. If you let the weeds get ahead of the crop, you can forget flame weed control. In general, flame weed control should be considered as an adjunct to a program of good mechanical cultivation. The mechanical cultivation can keep everything clean except the actual drill area, and small weeds there can be controlled with flame. It is important that any cultivations done before flaming leave the bed surface smooth and level; a rough condition will cause the flame to bounce up into the crop.

Running the Flame Weeder
It is difficult to see the flames in bright daylight, thus it is difficult to detect flameouts. The flames are easy to see at night (Figure 2). A more important reason to consider night operation is that research by the LSU AgCenter has found that some weeds are more susceptible to flame damage when they are turgid and even dew-wet, whereas they are not as easily killed during the day when they are somewhat limp and wilted. Operating speed must be low enough to allow the weeds to be heated.

Pre-emergence Flame Weed Control
Although the above discussion has dealt with post-emerge flame as an adjunct to crop cultivation, flame can also be used on a broadcast basis to kill small weeds on the bed top before planting. In that case, the flame burners can be mounted under a hood to contain and concentrate the flame. Water shielding is not needed for that operation.

In summary, flame weed control can be an excellent adjunct to cultivation for some crops and some weeds. Research by the LSU AgCenter has shown that beans and peas can be successfully grown without herbicides when timely mechanical cultivation and flame weed control are used.